Synchronizing devices in a wireless communication network

ABSTRACT

The present disclosure relates to a method for operating a base station of a wireless communication network. Precoding information for concentrating a radio-frequency power of a transmitted radio-frequency signal to a group of terminal devices arranged in a sub area of a coverage area of the base station is determined and applied to an antenna array of the base station. A synchronization signal is transmitted from the base station to the group of terminal devices using the precoding information.

CROSS REFERENCE TO RELATED APPLICATION

This application is a 35 U.S.C. § 371 national stage application of PCTInternational Application No. PCT/EP2017/078123, filed on Nov. 3, 2017,which claims priority from European Application No. EP 16 197 353.2filed on Nov. 4, 2016 the contents of which are incorporated herein byreference in their entireties. The above-referenced PCT InternationalApplication was published in the English language as InternationalPublication No. WO 2018/083208 A1 on May 11, 2018.

FIELD OF THE INVENTION

The present invention relates to a method for operating a base stationof a wireless communication network, in particular for synchronizingterminal devices of the wireless communication network.

BACKGROUND OF THE INVENTION

An increasing amount of devices may take advantage of communication in awireless communication network. For example, terminal devices referredto as Internet of Things (IoT) device or terminal devices using acommunication among each other, a so-called Machine Type Communication(MTC), may utilize communication via a wireless communication network.

Multiple-input and multiple-output systems, so-called MIMO systems, andin particular so-called massive MIMO systems, may improve spectralefficiency as well as energy efficiency by the use of channel estimationfor both uplink communication from a terminal device to a base stationand downlink communication from the base station to the terminal device.MIMO systems may use multiple send and receive antennas for wirelesscommunication at the base station as well as the terminal devices. Inparticular, in massive MIMO systems, the base station may include alarge number of antennas, for example several tens or even in excess ofone hundred antennas with associated transceiver circuitry. The extraantennas of the massive MIMO base station allow radio energy to bespatially focused in transmissions as well as a directional sensitivereception. In order to adapt and receive signals at each individualantenna of the base station in accordance with the currently activeterminal devices, the base station logic needs information aboutwireless radio channel properties between the terminal device and theantennas of the base station. A pilot signaling scheme, a so-calledchannel sounding, may be utilized for this purpose. The channel soundingallows the base station to set configuration antenna parameters fortransmitting signals so as to focus radio energy at the terminal deviceand for receiving signals from the terminal devices. Training sequences,so-called pilot signals, may be transmitted from the terminal device ina resource which is dedicated to the terminal device.

The massive MIMO approach may be utilized in particular in connectionwith IOT devices or MTC as the antenna gain is large and thus the powerconsumption for transmitting data of the terminal devices may bereduced. Furthermore, many IOT devices, for example smart meters, arestationary devices which may not be moved during their entire lifetime.Thus, the channel properties between an IOT device and the base stationmay not varying rapidly.

However, for setting up a wireless link between an base station and aterminal device, a temporal synchronization between the base station andthe terminal device may be needed and maintained. In particular, atemporal synchronization may already be needed for transmitting pilotsignals for estimating the radio channel properties in MIMO systems.Corresponding synchronization signals or messages may be defined in thewireless communication network. Nevertheless, an initialization of thewireless link between the MIMO base station and the terminal device,including for example a transmission of a synchronization signal andpilot signals, may not benefit from the enhanced antenna gain. Inparticular, synchronization and pilot signal transmission of terminaldevices arranged in areas having poor receiving conditions may raiseproblems.

SUMMARY OF THE INVENTION

Therefore, a need exists for advanced techniques for synchronizing basestations and terminal devices communicating via a wireless link. Inparticular, there is a need for techniques of synchronizing basestations and terminal devices communicating on a wireless link whichovercomes or mitigates at least some of the above identifiedrestrictions and drawbacks.

This need is met by the features of the independent claims. The featuresof the dependent claims define embodiments.

According to an embodiment, a method for operating a base station of awireless communication network is provided. The base station comprisesan antenna array comprising a plurality of antennas configured tocommunicate wirelessly with terminal devices positioned within acoverage area of the base station. A spatial transmission characteristicof the antenna array is configurable by applying precoding information.For example, to each antenna of the plurality of antennas of the antennaarray a corresponding transceiver is assigned. The precoding informationis applied to the antenna array by applying to each transceivercorresponding gain information provided by the precoding information.Additionally or as an alternative, to each transceiver a correspondingphase information provided by the precoding information is applied.

According to the method, precoding information is determined which isconfigured to concentrate a radio-frequency power of the transmittedradio-frequency signals to a group of terminal devices which is arrangedin a sub area of the coverage area of the base station. The group ofterminal devices may comprise at least one terminal device or aplurality of terminal devices. The sub area may comprise a sector of thecoverage area of the base station or a contiguous or noncontiguous areaof any shape, which may result from applying the precoding information.However, the area of the sub area may be narrower or smaller than thecoverage area of the base station. Therefore, when a certaintransmission power is emitted from the plurality of antennas using theprecoding information, the radio-frequency power received within the subarea is larger than the radio-frequency power received within the subarea when the same certain transmission power is transmitted from theplurality of antennas without using the precoding information.

Furthermore, according to the method, the determined precodinginformation is applied to the antenna array and a synchronization signalis transmitted from the base station to the group of terminal devicesusing the precoding information.

The synchronization signal may comprise a sequence of synchronizationsignals which may be averaged by each of the terminal devices to reducedisturbances on the synchronization signal and to acquiresynchronization to the base station.

In other words, a synchronization distribution in a sub area isperformed. This distribution is based on a beamforming of asynchronization signal into a sub area. Such a beamforming ofcorresponding synchronization signals into further sub areas may beconsecutively performed to cover all areas of the coverage area of thebase station or to cover at least all those areas where terminal devicesto be synchronized are arranged. Due to the beamforming a power of thesynchronization signal for the terminal devices located within the subarea may be enhanced. Under critical reception conditions, a terminaldevice may average several synchronization signals to synchronize to thebase station. With the enhanced power of the synchronization signal theterminal device does not need to average as much and may save someenergy and thus minimize energy consumption. Furthermore, thesynchronization signal distributed by the beamforming may be received bya plurality of terminal devices arranged within the sub area. Therefore,synchronization of a plurality of terminal devices may be initiated andmaintained with reasonable effort.

According to an embodiment, the precoding information is determinedbased on the corresponding position information of each terminal deviceof the group of terminal devices. For example, position information ofthe terminal devices may be provided by an application layer of thewireless communication network where this information is determinedbased on previous communication with the terminal devices. The positioninformation may comprise or may be derived from a geographical positionof the terminal device. Based on the position information, a grouping ofthe terminal devices may be conducted and precoding information may bedetermined such that a corresponding sub area covers the group ofterminal devices. The position information may comprise a velocity ofthe terminal device and/or moving direction of the terminal device.Based on the velocity and/or moving direction the base station maydetermine that a terminal device may not be synchronized by abeam-formed synchronization signal, as it is not stationary or movingtoo fast or will be moving out of the beam.

Additionally or as an alternative, the precoding information may bebased on corresponding previous precoding information, which has beenused during previous communications between the base station and eachterminal device of the group of terminal devices. In particular forstatic terminal devices, which are stationary or which do not movesignificantly, a once determined precoding information for distributinga synchronization signal may be reused for maintaining synchronization.

Additionally or as an alternative, the precoding information may beselected from a predefined set of precoding information based on a pilotsignal received from a terminal device. For example, a plurality of subareas may be defined and to each sub area a corresponding precodinginformation from the predefined set of precoding information isassigned. A pilot signal may be received from a terminal device andbased on the pilot signal the base station may associate the terminaldevice with a corresponding one of the plurality of sub areas. Thesending terminal device is assigned to the group of terminal devicesassigned to the sub area and corresponding precoding information. Forsynchronizing the terminal device, the corresponding precodinginformation is selected and applied to the antenna array for beamformingthe synchronization signal to the terminal device.

Furthermore, additionally or as an alternative, a predefined set ofprecoding information may be provided and each precoding information ofthe set may be assigned to a certain sub area such that by the whole setof precoding information the whole coverage area of the base station iscovered. Each precoding information of the set of precoding informationmay be selected and applied to the antenna array according to apredefined scheme and synchronization signals may be transmitted fromthe base station to the group of terminal devices using the currentlyselected precoding scheme. Each terminal device may listen to theemitted synchronization signals and may select the most appropriatesynchronization signal for synchronizing to the base station. Thesynchronization signals may be distributed into the sub areas and may beassociated with a resource block or identifier (ID). Each terminaldevice may receive one or more of these precoded synchronization signalsand may report to the base station which synchronization signal has thestrongest level in the sub area in which the corresponding terminaldevice resides.

According to another embodiment, a time interval information is receivedfrom at least one terminal device of the group of terminal devices. Thetime interval information indicates a required maximum time interval forsynchronizing the at least one terminal device. A schedule informationindicating a schedule for transmitting the synchronization signal usingthe precoding information is determined by the base station. Theschedule information may be determined based on the received timeinterval information such that the at least one terminal device issynchronized within the required maximum time interval. Furthermore,time interval information may be received from a plurality or all of theterminal devices of the group of terminal devices and the scheduleinformation may be determined based on the time interval informationreceived from these terminal devices to guarantee that each terminaldevice of the group of terminal devices is synchronized within therequired time interval.

The determined schedule information may be transmitted from the basestation to the group of terminal devices. The terminal devices of thegroup of terminal devices may use the schedule information received fromthe base station to wake up at corresponding points in time to receivethe synchronization signal for maintaining synchronization to the basestation.

Transmitting the time interval information from a terminal device to thebase station may include that the terminal device requests a schedulingscheme from the base station. The scheduling scheme may comprise a setof predetermined synchronization intervals or rates and the terminaldevice selects one of the intervals or rates, which is then used by thebase station for synchronizing the group of terminal devices.

According to another embodiment, timing and/or frequency information ofthe base station is acquired at a terminal device of the group ofterminal devices based on the synchronization signal transmitted fromthe base station. A pilot signal is transmitted from the terminal deviceto the base station based on the acquired timing and/or frequencyinformation. The pilot signal is configured for a channel sounding of aradio frequency channel between the terminal device and the basestation. Based on the channel sounding, the base station may determinecharacteristics of the radio-frequency channel. In other words, theterminal device may be synchronized to the base station and an uplinkpilot signal from the terminal device to the base station may betransmitted at a predefined timeslot and frequency without disturbingother communications within a radio-frequency cell supported by the basestation. Based on the received pilot signal, the base station maycommunicate with the terminal device according to MIMO or massive MIMOtechnologies.

In particular, the pilot signal from the terminal device may be receivedat the base station and the base station may determine further precodinginformation based on the pilot signal received from the terminal device.The further precoding information may be applied to the antenna array,and a synchronization signal may be transmitted from the base station tothe terminal device using the further precoding information.Additionally, payload data may be transmitted from the base station tothe terminal device using the further precoding information. Thus,synchronization signals and payload data may be transmitted from thebase station to the terminal device according to MIMO or massive MIMOtechnologies. This enables the synchronization signal to be transmittedwith the full MIMO gain such that a power consumption in the terminaldevice for receiving and averaging synchronization signals may bereduced.

According to a further embodiment, time interval information is receivedfrom the terminal device. The time interval information indicates arequired maximum time interval for synchronizing the terminal device.Further schedule information indicating a schedule for transmitting thesynchronization signal using the further precoding information isdetermined. As the further precoding information enables to focus atransmission to an individual terminal device, a time interval forupdating synchronization of the individual terminal device may bescheduled individually for each terminal device.

The further schedule information may be transmitted from the basestation to the terminal device.

According to the present invention, a base station is providedcomprising an antenna array and a control unit. The antenna array has aplurality of antennas configured to communicate wirelessly with terminaldevices positioned with in a coverage area of the base station. Aspatial transmission characteristic of the antenna array thisconfigurable by applying precoding information. The control unit isconfigured to determine precoding information for concentrating aradio-frequency power of transmitted radio-frequency signals to a groupof terminal devices arranged in a sub area of the coverage area of thebase station. Furthermore, the control unit is configured to apply thedetermined precoding information to the antenna array and to transmit asynchronization signal using the precoding information. For example, toeach antenna of the plurality of antennas of the antenna array acorresponding transceiver is assigned. The precoding information may beapplied to the antenna array by applying to each transceivercorresponding gain information provided by the precoding information orby applying corresponding phase information provided by the precodinginformation. By applying the precoding information, radio-frequencysignals transmitted from the antenna array may be beam-formed toincrease the received radio frequency power in the corresponding subarea. The synchronization signal may comprise a sequence of samesynchronization signals, which may be averaged by of the terminaldevices to reduce disturbances on the synchronization signal. Forexample, the synchronization signal may comprise a sequence of severaltens or some hundred same synchronization signals.

Determining the precoding information may comprise for example todetermine the precoding information based on corresponding positioninformation of each terminal device of the group of terminal devices.The position information of each of the terminal devices may be providedby an application layer of the wireless communication network. Theposition information may comprise for example a geographical position ofthe terminal device, the velocity and/or moving direction of theterminal device. Furthermore, the precoding information may bedetermined based on corresponding previous precoding information, whichhas been used during previous communications between the base stationeach terminal device of the group of terminal devices. Additionally oras an alternative, the precoding information may be selected from apredefined set of precoding information based on a pilot signal receivedfrom a terminal device. A plurality of sub areas may be defined and acorresponding precoding information may be assigned to each sub area.When a pilot signal is received from a terminal device, based on thepilot signal the base station may associate the terminal device to acorresponding one of the plurality of sub areas. The precodinginformation assigned to the sub area assigned to the terminal device isselected and applied to the antenna array for transmitting thesynchronization signal to the terminal device.

According to an embodiment, the base station may receive a time intervalinformation from at least one terminal device of the group of terminaldevices. The time interval information indicates a required maximum timeinterval for synchronizing the at least one terminal device. A scheduleinformation indicating a schedule for transmitting the synchronizationsignal using the precoding information is determined by the control unitof the base station based on the received time interval information. Thedetermined schedule information may be transmitted from the base stationto the group of terminal devices.

According to a further embodiment, the base station may receive a pilotsignal transmitted from the terminal device. The pilot signal may betransmitted by the terminal device based on timing and/or frequencyinformation acquired at the terminal device from the synchronizationsignal. The pilot signal may be configured for performing a channelsounding of a radio frequency channel between the terminal device andthe base station. Based on the channel sounding, the base station maydetermine characteristics of the radio-frequency channel. Based on thedetermined radio characteristics, the base station may determineprecoding information for transmitting radio-frequency signals to theterminal device according to MIMO or massive MIMO technologies. Inparticular, the base station may apply the determined precodinginformation and may transmit synchronization signals using the precodinginformation. Thus, a signal power of the synchronization signal receivedat the terminal device may be further increased.

According to another embodiment, the base station receives a timeinterval information from the terminal device. The time intervalinformation indicates a required maximum time interval for synchronizingthe terminal device. Based on the received time interval information,the control unit of the base station determines further scheduleinformation indicating a schedule for transmitting the synchronizationsignal using the further precoding information to the terminal device.

The further schedule information is transmitted from the base station tothe terminal device to enable the terminal device to wake up or activateits receiver for receiving synchronization signals.

Furthermore, according to the present invention, a terminal devicecomprising at least one antenna and a synchronization control unit isprovided. The at least one antenna is configured to wirelesslycommunicate with a base station of a wireless communication network. Thesynchronization control unit is configured to receive a sequence ofsynchronization signals from the base station, and to acquire timingand/or frequency information of the base station based on thesynchronization signals. For example, upon synchronization is required,the synchronization control unit may transmit time interval informationto the base station which indicates a required maximum time interval forsynchronizing the terminal device.

Furthermore, the synchronization control unit may receive scheduleinformation from the base station indicating for example timinginformation when the base station will transmit synchronization signals,which may be used by the terminal device to update and maintain itssynchronization to the base station. The synchronization control unitmay automatically change from an inactive state into an active statebased on the received schedule information. Thus, the terminal devicemay reduce the power required for maintaining and updatingsynchronization.

According to the present invention, a system is provided which comprisesthe above-described terminal device and the above-described basestation. The terminal device and the base station may be configured toperform the above-described method or anyone of the embodimentsdescribed above and comprises therefore also the above-describedadvantages.

Although specific features described in the above summary and thefollowing detailed description are described in connection with specificembodiments and aspects of the present invention, it should beunderstood that the features of the exemplary embodiments and aspectsmay be combined with each other unless specifically noted otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail with reference tothe accompanying drawings.

FIG. 1 shows schematically a base station and a terminal deviceaccording to embodiments of the present invention.

FIG. 2 shows schematically a system comprising a base station and aplurality of terminal devices according to embodiments of the presentinvention.

FIG. 3 shows schematically a system according to embodiments of thepresent invention.

FIG. 4 shows schematically method steps of a method according toembodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, exemplary embodiments of the present invention will bedescribed in more detail. It is to be understood that the features ofthe various exemplary embodiments described herein may be combined witheach other unless specifically noted otherwise. Same reference signs inthe various drawings refer to similar or identical components. Anycoupling between components or devices shown in the figures may be adirect or indirect coupling unless specifically noted otherwise.

FIG. 1 shows schematically a wireless cellular multiple-input andmultiple-output (MIMO) communication system comprising a base station 10and a terminal device 20. The base station 10 comprises an antenna array11 including a plurality of antennas 13. The base station 10 may have alarge number of antennas 13, such as several tens or in excess of onehundred antennas. The antennas 13 may be arranged in a two- orthree-dimensional spatial array on a carrier. The plurality of antennasmay also be spatially distributed to various locations, for example in acooperative MIMO system. Furthermore, it is possible that several basestations interact in a cooperative manner with the plurality of antennasbeing distributed over various locations. The communication system maybe a 3GPP-specified network such as 3G, 4G or upcoming 5G.

The base station 10 comprises a control unit (CU) 12 controllingcommunication with the terminal device 20 via the antenna array 11. Thecontrol unit 12 may be coupled to an application server (AS) 15, whichmay provide additional information about the terminal device 20, forexample a position information of the terminal device 20, an identifierof the terminal device 20 and a type of the terminal device 20, forexample if the terminal device 20 is a stationary device or a movabledevice, an IOT (Internet of things) device or a device using andsupporting machine type communication (MTC). The application server 15may be a separate device located apart from the base station 10 andcoupled to the base station 10 via a corresponding communication link,or the application server 15 may be integrated in the base station 10.For example, the application server can be a Mobile Edge Computing (MEC)server.

The base station 10, in particular the control unit 12 of the basestation 10, may be configured to transmit synchronization signals toterminal devices arranged within a coverage area of the base station 10to synchronize the terminal devices to the base station 10.

The plurality of antennas 13 of the antenna array 11 enables the basestation 10 to form a shape of a beam of a transmitted radio-frequencysignal. This technique, which is also called beamforming, increases thefield strength in some areas, whereas the field strength in some otherareas is lowered. The beamforming may be controlled by configuring anamplitude and phase of the transmitted radio-frequency signal for eachantenna 13 of the antenna array 11. By controlling the amplitude andphase a direction and shape may be configured. Configuring the amplitudeand phase of the antenna array 11 is also called precoding and thereforea radio frequency signal transmitted with certain amplitude and phaseconfiguration of the antenna array 11 is called a precoded radiofrequency signal.

In other words, the beamforming enables to concentrate the availabletransmission power into a certain sub area of the coverage area of thebase station 10. This increases a field strength in this sub area,whereas the field strengths in the remaining sub areas is lowered. Forexample, the coverage area of the base station may be divided into aplurality of sub areas in the form of sectors and for each sector acorresponding precoding may be provided to increase the field strengthwithin this sector significantly when being applied. FIG. 2 shows acoverage area 30 of the base station 10 divided into nine sectors 31 to39. Terminal devices 41 to 43 may be arranged in sector 37 and aterminal device 44 may be arranged in sector 38.

However, due to shadowing and reflections, the sub areas may have a morecomplex shape than the sectors shown in FIG. 2 and may additionally benoncontiguous. FIG. 3 shows an example of sub areas having a morecomplex shape. A coverage area 50 of the base station 10 is divided intofive sub areas 51 to 55 with the areas 51 and 54 being noncontiguous.Therefore, in FIG. 3, sub area 51 is indicated by reference signs 51Aand 51B, and sub area 54 is indicated by reference signs 54A and 54B.

The following methods for distributing synchronization signals in acoverage area of a base station will be described in connection with thesectors shown in FIG. 2 for clarity reasons. However, the principles maybe applied likewise to sub areas having a more complex shape as the subareas shown in FIG. 3.

The base station 10 is furthermore configured to analyze a pilot signalreceived from a terminal device at the plurality of antennas 13 of thebase station 10 to determine channel characteristics for a radio signaltransmission between the plurality of antennas 13 of the base station 10and the terminal device 20. For illustration, the control unit 12 of thebase station 10 may be configured to determine a footprint matrix basedon a pilot signal received by the plurality of antennas 13 from aterminal device 20. The control unit 12 may use the footprint matrix tocontrol the plurality of antennas 13 when transmitting radio frequencysignals to the terminal device 20. The control unit 12 may compute aHermitian conjugate of the footprint matrix to determine time delays andamplitudes of radio frequency signals transmitted by each of theplurality of antennas 13 to focus a radio energy in a sector in whichthe terminal device 20 is located. For example, the control unit 12 maycontrol a transmission of signals via the plurality of antennas 13 basedon radio channel properties, for example based on a footprint matrix,obtained for a pilot signal transmission from a terminal device 20 tocontrol a transmission of signals by the plurality of antennas 13 totransmit a downlink (DL) signal carrying control signaling or payloaddata to a spatial sector in which the terminal device 20 is located. Thecontrol may be performed in such a way that focusing of radio energy isnot only performed as a function of direction, but also as a function ofdistance from the base station 10. For example, for several terminaldevices located in the same direction and at similar distance from thebase station, the base station 10 may control the transmission ofsignals trough the plurality of antennas 13 in such a way that the radioenergy is focused in the direction in which the terminals are locatedand at the distance at which the terminals are located.

The terminal device 20 comprises at least one antenna 21, asynchronization control unit 22 and a control unit 23. The terminaldevice 20 may be for example a mobile telephone, in particular aso-called smart phone, or an Internet of things device. The control unit23 controls operation of the terminal device, for example, the controlunit 23 may execute applications provided in a memory of the terminaldevice 20. The terminal device 20 may comprise further components, forexample a user interface or sensors, a power supply, for example abattery, and a transceiver coupled to the antenna 21. However, theseadditional components are not shown in FIG. 1 for clarity reasons. Theterminal device 20 may be configured to communicate via a wirelesscommunication with the base station 10. For example, the terminal device20 may communicate voice data or sensor data with the base station 10.For operating the terminal device 20 in connection with the base station10 in for example a cellular wireless communication network, theterminal device 20 may have to be synchronized to a timing of the basestation 10, for example for receiving downlink (DL) data from the basestation 10 and transmitting uplink (UL) data to the base station 10.Synchronizing the terminal device 20 to the base station 10 may compriseadapting an internal timing of the terminal device 20 in frequency andphase to a timing of the base station 10.

In the following, examples for enabling a synchronization between a basestation and terminal devices communicating on a wireless link areprovided. A synchronization between the base station and terminaldevices enables to provide a common time reference and/or a commonfrequency reference to the base station and the terminal devicescommunicating on the wireless link. Based on such a common timereference and/or frequency reference, it is possible to implement atime-frequency resource mapping to synchronize communication of uplinksignals and/or downlink signals between the base station and terminaldevices communicating on the wireless link.

For example, a sequence of downlink synchronization signals may betransmitted from the base station 10 to the terminal device 20. Thesequence of synchronization signals may comprise multiple repetitions ofsynchronization signals and may have a well defined duration. Thesequence may use adjacent transmission frames of the wireless link fortransmission of the downlink synchronization signals. The sequence mayuse adjacent resources for transmission of downlink synchronizationsignals. In the terminal device 20, the received downlinksynchronization signals of the sequence may be averaged to increase asignal-to-noise ratio. This may enhance coverage of the base station 10.For example, averaging may correspond to summation of the power ormagnitude of the received downlink synchronization signals.

In wireless communication networks, synchronization accuracy maydeteriorate for example due to a movement of a terminal device withrespect to a base station, changes in the environment or due to adrifting of a timing circuit of the terminal device. In particular,Internet of things devices may be stationary such that synchronizationaccuracy is essentially influenced by the timing circuit drift and/orphysical changes of the environment. Therefore, in stationary terminaldevices an evaluation of synchronization signals from the base stationmay only be needed in certain time intervals to maintain synchronizationto the base station. The time intervals may be in the range of severalseconds or minutes up to several hours or even days. On the other hand,a stationary terminal device may have very restricted energy resources.For example, a power meter, which is remotely controllable and providessensor data via a wireless communication link, may be required to bepowered by a battery for several years. Consequently, receiving asequence of synchronization signals and averaging the synchronizationsignals may consume a considerable amount of electrical energy in viewof the available electrical energy from the battery. Therefore, thefollowing method described in connection with FIG. 4 may contribute toreduce the power consumption for synchronizing a terminal device to abase station.

The method described in FIG. 4 comprises a plurality of method steps 101to 112 which will be described in the following in connection with FIG.2. In step 101 the base station 10 may transmit omnidirectionalsynchronization signals. For example, the synchronization signals may betransmitted without any precoding such that the synchronization signalsmay be received within the whole coverage area 30. In particular mobileterminal devices and newly established terminal devices may receivethese omnidirectional synchronization signals and may synchronize to thebase station based on the received synchronization signals in step 102.Terminal devices at the periphery of the coverage area 30 may need toaverage the synchronization signals over a long time to synchronize andthis may have an impact on the power consumption needed forsynchronization.

In particular, stationary devices may indicate they are position or howto be reached more efficiently to the base station, for example via theapplication server 15. Based on this information the base station 10 maydetermine precoding information for concentrating a transmission ofsynchronization signals into sub areas in step 103. For example,terminal devices 41 to 43 may transmit their geographical positions tothe base station 10 and the base station 10 may determine acorresponding precoding information concentrating a radio-frequencypower of a synchronization signal into sub area or sector 37. In step104 the base station applies the determined precoding information forthe sub area to the antenna array 11 and transmits synchronizationsignals concentrated into the sub area in step 105. For example, acertain frequency and/or time resource may be used for transmitting aprecoded synchronization signal into sector 37.

In another example, the base station may provide a plurality ofprecoding information, which cover as a whole essentially the wholecoverage area of the base station. Thus, synchronization signals may bedistributed into all sectors and may be associated with a resource blockor identifier (ID). Each terminal device may receive one or more ofthese precoded synchronization signals and may report to the basestation which synchronization signal has the strongest level in thesector in which the corresponding terminal device resides.

Additionally or as an alternative, terminal devices 41-43 may transmiteach a pilot signal to the base station 10, and the base station 10 maydetermine from the pilot signals corresponding precoding matrixes. Basedon the precoding matrixes, the base station 10 may determine a precodinginformation which defines a sub area which covers all of the terminaldevices 41 to 43. For example, a specific precoding information may becalculated based on the precoding matrixes. As an alternative, a set ofpredefined precoding information may be provided and the terminaldevices may be assigned to a corresponding one of the correspondingprecoding information based on a matching of the precoding matrix to theprecoding information. The transmission of the synchronization signalsby beamforming the synchronization signals to the certain sector or subarea, results in an enhanced synchronization signal for terminal deviceslocated within this sub area. For example, a terminal device within thissub area does not need to average as much and can save therefore energyfor synchronization and thus minimize energy consumption.

Furthermore, a scheduling of distribution of synchronization signals maybe established. For example, a terminal device may communicate to thebase station a time interval, which is not to be exceeded before a nextsynchronization has to be performed to maintain the terminal devicewithin a required synchronization accuracy to the base station (step106). Based on the time interval from the terminal device or based onseveral time intervals received from several terminal devices within thesame sub area, the base station may determine a time schedule fortransmitting precoded synchronization signals focused into the sub area.The time schedule may be transmitted to the terminal devices within thesub area in step 107.

As an alternative, the base station may provide a plurality of frequencyand/time resources for transmitting synchronization signals and acorresponding time schedules indicating when synchronization signalswill be provided in the corresponding resources. A terminal devicearranged in the sub area may select an appropriate resource formaintaining synchronization to the base station. Based on the timeschedule the terminal devices may wake up the synchronization controlunit 22 only when a synchronization update to the base station isrequired and the synchronization signals are provided by the basestation. A wake-up a rate may be in a range of minutes, hours or evenonce a day.

For further enhancing the synchronization of a terminal device to thebase station 10, a channel sounding of a radio-frequency channel betweenthe base station 10 and a terminal device may be performed in step 108.For example, the terminal device, for example terminal device 44 in FIG.2, may transmit a pilot signal to the base station 10. The base station10 may receive the pilot signal from the terminal device 44 anddetermine, based on the pilot signal, precoding information whichfocuses a radio frequency signal individually to the terminal device 44in step 109. The precoding information may be applied to the antennaarray 11 of the base station 10 and a synchronization signal focused onthe individual terminal device 44 may be transmitted using the precodinginformation in step 110.

The base station and the terminal device 44 may negotiate a timeinterval for the transmission of synchronization signals precodedindividually for the terminal device 44 in steps 111 and 112. Forexample, the terminal device 44 may transmit a time interval informationto the base station, which indicates a required maximum time intervalfor synchronizing the terminal device 44 (step 111). The base station 10receives the time interval information and determines a correspondingschedule information for transmitting the synchronization signal atcertain frequencies/time resources using the precoding information,which directs the synchronization signal specifically to the terminaldevice 44. The schedule information is transmitted in step 112 to theterminal device 44, and the terminal device 44 automatically changesfrom an inactivity state into an active state based on the receivedschedule information to receive the synchronization signal at thecertain frequency/time resources. In other words, the synchronizationsignal is precoded according to (massive) MIMO technologies and acorresponding (massive) MIMO gain may be gained for the transmission ofthe synchronization signal. This may be in particular interesting forlow-cost terminal devices where the internal timing may have a lowperformance and averaging over a long period of time may be a challenge.Furthermore, due to the shorter averaging, electrical energy forreceiving the synchronization signal and synchronizing the internaltiming may be reduced.

According to the schedule, the precoded synchronization signals may betransmitted periodically. However, the precoded synchronization signalsmay be transmitted aperiodically, for example based on requests from theterminal device.

Although the steps 101 to 112 are shown and described above in asubsequent order, these steps may be performed in any other order. Inparticular, the steps 101 to 112 may be performed in parallel tosimultaneously synchronize newly established and mobile terminal deviceswith omnidirectional synchronization signals (steps 101, 102), and tomaintain synchronization of stationary terminal devices, like Internetof things devices, with precoded synchronization signals which areconcentrated on certain sub areas (step 105) and which are directedindividually to a certain terminal device (step 110). Furthermore,according to an example, for some reasons, for example loss of beamalignment, the terminal device is unable to be synchronized or toprocess a beam-formed synchronization signal. In this case, the terminaldevice may need to switch back to “listening” to the omnidirectionalsynchronization signal provided in step 101.

The invention claimed is:
 1. A method for operating a base station of awireless communication network, the base station comprising an antennaarray having a plurality of antennas configured to communicatewirelessly with terminal devices positioned within a coverage area ofthe base station, a spatial transmission characteristic of the antennaarray being configurable by applying precoding information, the methodcomprising: determining the precoding information for concentrating aradio frequency power of a transmitted radio frequency signal to a groupof terminal devices arranged in a sub area of the coverage area of thebase station, applying the determined precoding information to theantenna array, transmitting, from the base station to the group ofterminal devices, a synchronization signal using the precodinginformation, receiving time interval information from at least oneterminal device of the group of terminal devices, the time intervalinformation indicating a required maximum time interval forsynchronizing the at least one terminal device, and determining scheduleinformation indicating a schedule for transmitting the synchronizationsignal using the precoding information.
 2. The method according to claim1, wherein determining the precoding information comprises at least oneof: determining the precoding information based on correspondingposition information of each terminal device of the group of terminaldevices, determining the precoding information based on correspondingprevious precoding information, which has been used during previouscommunications between the base station and each terminal device of thegroup of terminal devices, selecting the precoding information from apredefined set of precoding information based on a pilot signal receivedfrom a terminal device, wherein the terminal device is assigned to thegroup of terminal devices, or selecting the precoding information from apredefined set of precoding information based on a report received froma terminal device of the group of terminal device, wherein the reportindicates which precoding information provides a synchronization signalhaving the strongest level in the sub area in which the correspondingterminal device resides.
 3. The method according to claim 2, wherein theposition information comprises at least one of: a geographical positionof the terminal device, a velocity of the terminal device, or a movingdirection of the terminal device.
 4. The method according to claim 1,further comprising: transmitting the schedule information from the basestation to the group of terminal devices.
 5. The method according toclaim 1, further comprising: acquiring, at a terminal device of thegroup of terminal devices, timing and/or frequency information of thebase station based on the synchronization signal transmitted from thebase station, and transmitting a pilot signal from the terminal deviceto the base station based on the acquired timing and/or frequencyinformation, the pilot signal being configured for a channel sounding ofa radio frequency channel between the terminal device and base stationfor determining characteristics of the radio frequency channel.
 6. Themethod according to claim 5, further comprising: receiving the pilotsignal from the terminal device at the base station, determining furtherprecoding information based on the pilot signal received from theterminal device, applying the further precoding information to theantenna array, and transmitting, from the base station to the terminaldevice, the synchronization signal using the further precodinginformation.
 7. The method according to claim 6, further comprising:receiving, from the terminal device, time interval information, the timeinterval information indicating a required maximum time interval forsynchronizing the terminal device, and determining further scheduleinformation indicating a schedule for transmitting the synchronizationsignal using the further precoding information.
 8. The method accordingto claim 7, further comprising: transmitting the further scheduleinformation from the base station to the terminal device.
 9. The methodaccording to claim 1 corresponding transceiver is assigned, whereinapplying the precoding information comprises at least one of: applyingto each transceiver corresponding gain information provided by theprecoding information, or applying to each transceiver correspondingphase information provided by the precoding information.
 10. The methodaccording to claim 1, wherein the synchronization signal comprises asequence of synchronization signals.
 11. A base station, comprising: anantenna array having a plurality of antennas configured to communicatewirelessly with terminal devices positioned within a coverage area ofthe base station, a spatial transmission characteristic of the antennaarray being configurable by applying precoding information, and acontrol unit configured to determine precoding information forconcentrating a radio frequency power of a transmitted radio frequencysignal to a group of terminal devices arranged in a sub area of thecoverage area of the base station, to apply the determined precodinginformation to the antenna array, to transmit a synchronization signalusing the precoding information, to receive time interval informationfrom at least one terminal device of the group of terminal devices, thetime interval information indicating a required maximum time intervalfor synchronizing the at least one terminal device, and to determineschedule information indicating a schedule for transmitting thesynchronization signal using the precoding information.
 12. A terminaldevice, comprising at least one antenna configured to wirelesslycommunicate with a base station of a wireless communication network, asynchronization control unit configured to receive a sequence ofsynchronization signals from the base station, to acquire timing and/orfrequency information of the base station based on the synchronizationsignals, and to transmit time interval information to the base station,the time interval information indicating a required maximum timeinterval for synchronizing the terminal device.
 13. The terminal deviceaccording to claim 12, wherein the synchronization control unit isconfigured to receive schedule information from the base station, and toautomatically change from an inactive state into an active state basedon the received schedule information.